Various inkjet printing arrangements include both thermally actuated printheads and mechanically actuated printheads. Thermally actuated printheads tend to use resistive elements or the like to achieve ink expulsion, while mechanically actuated printheads tend to use piezoelectric transducers or the like.
A representative thermal inkjet printhead of a print cartridge has a plurality of thin film resistors provided on a semiconductor substrate. A barrier layer is deposited over thin film layers on the substrate. The barrier layer defines firing chambers about each of the resistors, an orifice corresponding to each firing chamber, and an entrance or fluid channel to each firing chamber. Often, ink is provided through a slot in the substrate and flows through the fluid channel to the firing chamber. Actuation of a heater resistor by a “fire signal” causes ink in the corresponding firing chamber to be heated and expelled through the corresponding orifice.
In order to provide high print quality, each nozzle (or orifice) of the printhead should be able to repeatably deposit the desired amount of ink in the proper pixel location on a medium, producing round spots or dots. However, printhead aberrations and the effects of aging can adversely affect ink drop placement. The actual location of misplaced drops can visibly differ from the desired location, much like missing the bulls-eye of a target. The location error can have a component in the direction in which the print cartridge is scanned; such error is known as scan axis directionality (“SAD”) error. The location error can also have a component in the direction in which the medium is advanced; such error is often called paper axis directionality (“PAD”) error.
Another form of drop placement error also occurs because fluid is typically not ejected from a nozzle in the form of a single drop, but rather as a main drop followed by one or more satellite drops. All of these drops would ideally be deposited in the same pixel location; however, because the main and satellite drops are ejected at slightly different times with slightly different velocities, satellite drops often land downstream in the scan direction from the main drop. Instead of printing a round spot on the medium, non-coincident main and satellite drops can produce a non-round spot with a “tail”, or even more than one spot on the medium. As the scanning speed of the printhead with respect to the medium increases, the time separation between the main and satellite drops has a greater effect, and it becomes more likely that the main and satellite drops will not result in round spots as desired.
Drop placement errors generally cause a visually significant print quality defect known as banding: strip-shaped nonuniformities that are visible throughout the printed image. Banding is particularly noticeable when the drop placement errors are not consistent from nozzle to nozzle on the printhead. Banding is also particularly noticeable when the drop placement errors for a single nozzle vary between consecutive drops, such as when the main and satellite drops sometimes coincide, but other times don't coincide. Furthermore, a combination of round and non-round spot shapes in an area on the medium which is intended to be printed with a uniform color and intensity can result in an undesirable variation of lightness and darkness within the supposedly uniform area. Accordingly, it would be desirable to deposit drops of fluid in a repeatably accurate and/or precise manner.